Sassuolo make a mess of clearing their lines from a corner, with the ball rebounding kindly to Ronaldo off the post for the 33-year-old to tap into an empty net.
Juve midfielder Douglas Costa was sent off in stoppage time after spitting at Federico Di Francesco having become involved in a running feud with the Sassuolo midfielder.
Sassuolo mustered a fightback and did get a consolation goal through Khouma Babacar in the 90th but it was not enough to deny Ronaldo and his teammates their fourth victory in as many games.
They will now turn their attention to Wednesday night and their first Champions League game of the season, away to Valencia.
The Turin giants also won the European title in 1985, but finished runners-up twice in 2015 and 2017 under current coach Allegri.
Manchester City revenue tops £500m and profit soars
David Silva reached a half-century of Premier League goals when he scored against Fulham - so look back at his best bits here. City begin their Champions League campaign against Olympique Lyon at the Etihad on Wednesday.
"It's a very hard group, we know that we can do well and Juve must focus on being the best".
Coincidentally, with Real Madrid, it took 28 shots for him to open his account, but there was never any doubt that a goal would come sooner or later.
His barren run had stretched beyond the 300-minute mark during a goalless first-half at the Allianz Stadium, but he did not have to wait long to make his mark after the interval.
"This was a game we had under control, we should have closed it down".
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